Titan: Land of lakes—and drizzle

A newly assembled mosaic of radar images of Saturn’s hydrocarbon-shrouded moon Titan, taken over the past 18 months by the Cassini spacecraft, shows what are probably hydrocarbon lakes and seas at the moon’s north pole. At least one of the lakes is larger than Lake Superior. In addition, radar images taken by Cassini during an Oct. 2 flyby show evidence of hydrocarbon lakes at the moon’s south pole.

Cassini’s radar has now studied 60 percent of the north polar region above 60° latitude. Lakes appear to occupy about 14 percent of the scanned area. Planetary scientists have long proposed that lakes could form because of methane and ethane raining down from the moon’s atmosphere. Cassini’s radar view of a small patch at the south pole shows three lakes, indicating that these features may be just as common there as at the north pole.

“All the circumstantial evidence points to the lakes being filled, partially or fully, with liquid, but the radar cannot give us direct positive evidence,” notes Jonathan Lunine of the University of Arizona in Tucson. “Confirmation will require difficult, near-infrared observations that might provide a signature of liquid methane or ethane,” he says.

In a separate finding, near-infrared studies using two Earth-based telescopes suggest that methane clouds are drizzling the hydrocarbon onto Titan. In an equatorial region called Xanadu, the observations show widespread methane cloud cover at high altitudes and also suggest the presence of drizzle lower in the atmosphere. The clouds and precipitation, which either strike the ground or turn into mist, seem to dissipate by mid-morning, as measured locally.

“Widespread and persistent drizzle may be the dominant mechanism for returning methane to the surface from the atmosphere and closing the methane cycle,” Máté Ádámkovics of the University of California, Berkeley and his colleagues note in an upcoming Science.